Long time no see!
The last few versions of ACR have given us Transform Guides. You use these to draw along actual horizontal and vertical edges to straighten perpective distortion. They're great.
My problem starts when I have spent a few minutes carefully lining these up on an image, and then decide to apply an ACR preset. The guides disappear and the distortion returns. This is still the case in ACR 9.8.
Is this a bug, or am I doing something wrong?
I believe you have uncovered a bug, or at least an inconsistency. I made a very simple preset, all it did was increase exposure one stop, all the other items were unchecked. I then tried several operations, including cropping, and except for the transform the preset could be applied without the image reverting to unchanged.
I drew in a couple of transform guides, enough to make a significant difference to the image. I then applied the preset, and the transformation disappeared. I then reapplied the guides, and added one stop of exposure without using a preset, all worked as expected, the transformation remained as the exposure was increased.